The present invention relates generally to synthetic aperture radar systems and, more specifically, to a synthetic aperture radar system for mapping a vehicle radar cross section to identify areas of significant scattering.
Synthetic aperture radar (SAR) has long been used to produce two-dimensional radar images of a target. The theory of SAR is well-known and described in Skolnik, Radar Handbook, McGraw-Hill, New York, 1970. Briefly, SAR increases the effective aperture size of the radar by moving the antenna relative to the target. This movement increases target resolution far beyond the resolution achievable with a stationary antenna of the same physical size.
SAR is most commonly used in airborne or spaceborne radars to produce high-resolution images of the ground or ocean. As the radar platform traverses a linear path, sequential amplitude and phase data are received from a single antenna and stored. Well-known signal processing techniques, such as two-dimensional Fourier transforms, are then used on the stored data to produce an image of the terrain over which the antenna has passed.
The resolution of the image depends upon the effective aperture size, L.sub.eff, which is the velocity of the aircraft divided by the time period over which the data is collected. The effective beamwidth of the antenna. .beta..sub.eff, is the wavelength, .xi., divided by twice the effective aperture, L.sub.eff. The azimuthal resolution of the image, .delta..sub.a, is equal to the effective beamwidth (.beta..sub.eff) multiplied by the range from the antenna to the ground, i.e., the altitude of the aircraft.
Resolution in the range or longitudinal dimension can be achieved by sweeping the radar pulses in frequency. The range resolution, .delta..sub.r is the velocity of the radar pulses, c, divided by twice the bandwidth of the frequency sweep.
SAR has also been used to form images of underground objects and of ships at sea. U.K. Patent 2,202,705 discloses a transmitting antenna and a receiving antenna disposed a fixed distance apart that move in unison over the surface of the ground in a direction perpendicular to the line between them. U.S. Pat. No. 4,723,124 issued to Boles describes a shipboard SAR having a phased array antenna. Reflections from target scattering centers are received and used to form an image of the target ship.
When SAR is used for imaging terrain, ships, underground objects, and other similar targets, it is desirable for the objects to backscatter a large portion of the radar energy. It is not possible, of course, to form a SAR image of an object that does not backscatter radar energy. The areas of the target having strong backscattering are known as "scattering centers."
In recent years, much effort has been directed towards producing aircraft that are difficult to detect using radar. These low-observability (LO) or "stealth" aircraft are difficult to detect because they employ various means to minimize the backscattering of radar energy.
The effectiveness of these means for reducing backscatter must presently be tested either while the aircraft is in flight or by measuring the backscatter from the aircraft with a stationary radar while rotating the aircraft. Moving a target relative to a stationary antenna is known as Inverse Synthetic Aperture Radar (ISAR), which can be used to produce a radar cross section image in which the scattering centers are clearly identifiable. Such ISAR test ranges must include a large turntable for supporting and rotating an aircraft to be imaged. Furthermore, the ISAR must image the aircraft in a large, open area or an anechoic chamber because the image of the aircraft will be obscured if the radar receives backscatter from points that are stationary relative to the radar. Thus, ISAR test ranges are neither easily transported nor quickly readied for use.
Scattering centers may be produced on the LO aircraft by patches of dirt, production defects, exterior damage, or incompletely closed access doors. Such conditions may go unnoticed by maintenance personnel and pilots in the field. Furthermore, repairs and production defects may leave imperfections that may not be detected by visual inspections. As a result, the aircraft may be vulnerable to radar detection. Unless the aircraft is brought to an ISAR test range, these conditions will often remain undetected.
A compact device for quickly and easily pinpointing scattering centers of an aircraft or other vehicle either in the field or on the factory floor would be highly desirable. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.